BaMn[CO3]2 – a previously unrecognized double carbonate in low-temperature environments: Structural,spectroscopic, and textural tools for future identification

Following a tentative evidence for the occurrence of low-temperature barium manganese(II) carbonates in brackish sediments of the Baltic Sea, a stoichiometric double carbonate, BaMn[CO₃]₂, was synthesized from aqueous solutions at ambient temperature for the first time. Here we report the results of a multi-method approach, including scanning electron microscopy (SEM–EDX) investigations, the chemical composition, and, in particular, the diagnostic powder X-ray diffraction pattern, as well as diagnostic parts of the FT infrared absorption and Raman spectra for future identification of this new carbonate in low-temperature environments, like brackish sediments.     

Late Quaternary (24–10 ka BP) environmental history of the Neotropical lowlands inferred from ostracodes in sediments of Lago Petén Itzá, Guatemala

We inferred late Pleistocene and early Holocene (24–10 ka BP) environmental conditions in and around Lago Petén Itzá, Guatemala from ostracode remains in the lake sediments. Multivariate statistics were run on autecological information for 29 extant ostracode species collected in 63 aquatic ecosystems on the Yucatán Peninsula along a steep, increasing NW–S precipitation gradient and across a large altitudinal range. Conductivity and water depth are the most important factors that shape ostracode communities. Transfer functions were developed and applied to fossil ostracode assemblages in a ~76-m sediment core (PI-6, ~85 ka) taken in 71 m of water from Lago Petén Itzá, to infer past shifts in conductivity and water level. Results suggest climate was cold and wet during the Last Glacial Maximum (LGM). Alternating dry and wet conditions characterized the deglacial. Early Holocene climate was warmer and wetter. The LGM was characterized by low ostracode species richness (4 spp.) and abundance (<940 valves g⁻¹), dominance of benthic over nektobenthic taxa, abundant Physocypria globula, conductivity as low as 190 μS cm⁻¹, and clay-rich sediments with relatively high total organic carbon and low C/N ratios (<14), suggesting relatively deeper water at the core site associated with abundant precipitation. Greatest water depth at the core site during the LGM occurred late in the period and was ~50 m. The deglacial was characterized by drier conditions, higher ostracode species richness (6 spp.) and abundances up to 18,115 valves g⁻¹, dominance of nektobenthic species, and presence of shallow-water and littoral-zone indicators such as Heterocypris punctata and Strandesia intrepida, conductivity up to 550 μS cm⁻¹, C/N ratios as high as 37, and gypsum deposition. Lowest inferred lake depth at the core site during the deglacial was ~20 m. The early Holocene was characterized by high numbers of ostracode remains, up to 25,500 valves g⁻¹, and the presence of L. opesta and P. globula. Cytheridella ilosvayi was absent from late Pleistocene sediments, suggesting it colonized northern Central America during the Holocene.     

Aseismic deformation in the Alps: GPS vs. seismic strain quantification

Neotectonics of the Western and Central Alps is characterized by ongoing widespread extension in the highest zones of the chain and transcurrent/compressive tectonics at the external limits of the belt. The overall geodetically measured deformations also indicate extension across the Western Alps. There is a good qualitative coherency between seismotectonic and geodetic approaches. Here we attempt to quantify the seismic part of the deformation. The seismic strain is compared to the deformation derived from geodesy. In sub‐areas of homogeneous seismic stress/strain, we computed the total seismic moment tensor and related strain tensor. This study provides new quantitative elements about the ongoing geodynamic processes in the alpine belt. The important discrepancies obtained between seismic strains and geodetically‐measured deformations raise the issue of aseismic deformation in the Alps, which could be related to elastic loading, creeping and/or a slower ductile‐style deformation.     

Impactites of the Yaxcopoil‐1 drilling site,Chicxulub impact structure: Petrography,geochemistry, and depositional environment

Abstract— The impact breccias encountered in drill hole Yaxcopoil‐1 (Yax‐1) in the Chicxulub impact structure have been subdivided into six units. The two uppermost units are redeposited suevite and suevite, and together are only 28 m thick. The two units below are interpreted as a ground surge deposit similar to a pyroclastic flow in a volcanic regime with a fine‐grained top (unit 3; 23 m thick; nuée ardente) and a coarse breccia (unit 4; ～15 m thick) below. As such, they consist of a mélange of clastic matrix breccia and melt breccia. The pyroclastic ground surge deposit and the two units 5 and 6 below are related to the ejecta curtain. Unit 5 (～24 m thick) is a silicate impact melt breccia, whereas unit 6 (10 m thick) is largely a carbonate melt breccia with some clastic‐matrix components. Unit 5 and 6 reflect an overturning of the target stratigraphy. The suevites of units 1 and 2 were deposited after emplacement of the ejecta curtain debris. Reaction of the super‐heated breccias with seawater led to explosive activity similar to phreomagmatic steam explosion in volcanic regimes. This activity caused further brecciation of melt and melt fragments. The fallback suevite deposit of units 1 and 2 is much thinner than suevite deposits at larger distances from the center of the impact structure than the 60 km of the Yax‐1 drill site. This is evidence that the fallback suevite deposit (units 1 and 2) originally was much thicker. Unit 1 exhibits sedimentological features suggestive of suevite redeposition. Erosion possibly has occurred right after the K/T impact due to seawater backsurge, but erosion processes spanning thousands of years may also have been active. Therefore, the top of the 100 m thick impactite sequence at Yaxcopoil, in our opinion, is not the K/T boundary.     

Active tectonism in the central Alps: contrasting stress regimes north and south of the Rhone Valley

An integrated interpretation of seismicity, fault plane solutions and deep seismic reflection data suggests that the NE–SW to NW–SE trending Rhone–Simplon fault zone and the gently S-dipping basal Penninic thrust separate fundamentally different stress regimes in the western Swiss Alps. North of the Rhone-Simplon fault zone, strike-slip earthquakes on steep-dipping faults within the Helvetic nappes are a consequence of regional NW–SE compression and NE–SW extension. To the south, vertical maximum stress and N–S extension are responsible for normal mechanism earthquakes that occur entirely within the Penninic nappes above the basal Penninic thrust. Such normal faulting likely results from extension associated with southward movements (collapse) of the Penninic nappes and/or continued uplift and relative northward displacements of the underlying Alpine massifs. Geological mapping and fission-track dating suggest that the two distinct stress regimes have controlled tectonism in the western Swiss Alps since at least the Neogene.     

Peak‐temperature patterns of polyphase metamorphism resulting from accretion,subduction and collision (eastern Tauern Window,European Alps) – a study with Raman microspectroscopy on carbonaceous material (RSCM)

Raman microspectroscopy on carbonaceous material (RSCM) from the eastern Tauern Window indicates contrasting peak‐temperature patterns in three different fabric domains, each of which underwent a poly‐metamorphic orogenic evolution: Domain 1 in the northeastern Tauern Window preserves oceanic units (Glockner Nappe System, Matrei Zone) that attained peak temperatures (T_p) of 350–480 °C following Late Cretaceous to Palaeogene nappe stacking in an accretionary wedge. Domain 2 in the central Tauern Window experienced T_p of 500–535 °C that was attained either within an exhumed Palaeogene subduction channel or during Oligocene Barrovian‐type thermal overprinting within the Alpine collisional orogen. Domain 3 in the Eastern Tauern Subdome has a peak‐temperature pattern that resulted from Eo‐Oligocene nappe stacking of continental units derived from the distal European margin. This pattern acquired its presently concentric pattern in Miocene time due to post‐nappe doming and extensional shearing along the Katschberg Shear Zone System (KSZS). T_p values in the largest (Hochalm) dome range from 612 °C in its core to 440 °C at its rim. The maximum peak‐temperature gradient (≤70 °C km^?1) occurs along the eastern margin of this dome where mylonitic shearing of the Katschberg Normal Fault (KNF) significantly thinned the Subpenninic‐ and Penninic nappe pile, including the pre‐existing peak‐temperature gradient.     

Towards a Higher Comparability of Geothermometric Data Obtained by Raman Spectroscopy of Carbonaceous Material. Part 2: A Revised Geothermometer

The estimation of the peak metamorphic temperature by Raman spectroscopy of carbonaceous material (RSCM) is influenced by several bias sources grouped in measuring conditions, spectral processing and sample heterogeneity. The measuring conditions (selected excitation wavelength) and the operator bias during spectral processing have a pronounced impact on the temperature estimate and thus on the comparability and portability of thermometric data obtained by RSCM. Several calibration lines of RSCM geothermometers are published already, but no standardised approach exists. Samples of carbonaceous material bearing metasediments with well‐established metamorphic conditions of the central and western Alps compile a reference series. By applying an automated, iterative and randomised curve‐fitting approach, a consistent and user input‐independent RSCM geothermometer is presented, which covers peak metamorphic temperatures from ca. 160 to 600 °C. The method is hardware independent because the measuring conditions bias is excluded by the use of the reference series and the automated curve‐fitting approach reduces the spectral processing bias effectively, increasing the method's comparability and portability. By distributing the reference series and the automated curve‐fitting software, a laboratory will be able to derive a laboratory specific calibration line for the RSCM geothermometer.     

The replacement of plagioclase feldspars by albite: observations from hydrothermal experiments

Oligoclase and labradorite crystals have been experimentally replaced by albite in an aqueous sodium silicate solution at 600°C and 2 kbars. The replacement is pseudomorphic and is characterised by a sharp chemical interface which progresses through the feldspar while preserving the crystallographic orientation. Reaction rims of albite, up to 50 μm thick, can be readily achieved within 14 days. Re-equilibration of plagioclase in an ¹⁸O-enriched sodium- and silica-bearing solution results in oxygen isotope redistribution within the feldspar framework structure. The observed characteristics of the reaction products are similar to naturally albitised plagioclase and are indicative of an interface-coupled dissolution–reprecipitation mechanism. Chemical analyses demonstrate that the albitisation is accompanied by the mobilisation of major, minor and trace elements also including elements such as Al and Ti which are commonly regarded as immobile during hydrothermal alteration. The results contribute to developing our understanding of the close association between large-scale albitisation and secondary ore mineralisation which is common in nature.     

Evidence for X-Ray Emission from a Large-Scale Filament of Galaxies?

The large-scale ( approximately 100 kpc) environments of Seyfert galaxies are not significantly different from those of non-Seyfert galaxies. In the context of the interaction model of the formation of active galactic nuclei (AGNs), it has thus been proposed that AGNs form via "minor mergers" of large disk galaxies with smaller companions. We test this hypothesis by comparing the nuclear spectra of 105 bright nearby galaxies with measurements of their R- or r-band morphological asymmetries at three successive radii. We find no significant differences between these asymmetries among the 13 Seyfert galaxies in the sample and galaxies having other nuclear spectral types (absorption, H ii region-like, LINER), nor is there strong qualitative evidence that such mergers have occurred among any of the Seyfert galaxies or LINERs. Thus, either any minor mergers began greater, similar1 Gyr ago and are essentially complete, or they did not occur at all, and AGNs form independently of any type of interaction. Support for the latter interpretation is provided by the growing evidence that supermassive black holes exist in the cores of most elliptical and early-type spiral galaxies, which in turn suggests that nuclear activity represents a normal phase in the evolution of the bulges of massive galaxies. Galaxy mergers may increase the luminosity of Seyfert nuclei to the level of QSOs, which could explain why the latter objects appear to be found in rich environments and in interacting systems.     

Dépliage 3-D des anticlinaux bordant le synclinal fermé de la vallée des Ponts, Jura central, Suisse

The Ponts valley syncline is a closed basin within the Neuchâtel Jura fold and thrust belt. This syncline, apparently uplifted to an altitude of around 1000m is closed in the SW by an anticline with an oblique WNW-ESE direction. The 3-D geometry of the entire structure is examined and unfolded in detail. This syncline is filled with an unexpectedly thick series (~400m) of Tertiairy Molasse, as revealed by the CS-AMT (controlled source audio-magneto-telluric) and a reflexion seismic line. The latter also documents internal compressional structures within the well layered upper freshwater Molasse series. The 3-D configuration of the top Malm limestones has been constructed for the entire area based on new detailed geologic and structural mapping, hundreds of dip measurements, as well as geophysical data. The Malm marker bed displays three distinct types of structures: 1) Thrust faults with shallow dips, vergent to the NW and/or SE that are associated with folds interpreted as fault bend folds; 2) high angle inverse faults, mostly with a SE vergence are interpreted as inverted normal faults, inherited from a modest Oligo-Miocene extensional phase in a NW-SE direction; and 3) tear faults with a dominant N-S direction, probably inherited from an Oligocene extensional phase in association with the opening of the Rhine and Bresse grabens. Tear faults accommodate important lateral changes in fold geometry during the Late Miocene main folding-and-thrusting phase. All deformations are easily explained in an entirely thin-skinned fashion, taking place above a thick detachment horizon within Triassic evaporite series.Manuscrit reçu le 31 mars 2003 Révision acceptée le 23 juin 2004